The present invention relates to a process for attenuating the yawing moment in a vehicle with an anti-lock braking system (ABS). The present invention is related to the inventions disclosed in commonly assigned co-pending applications Ser. No. 08/756,593, filed Nov. 27, 1996, and Ser. No. 08/854,289, filed May 9, 1997.
When a vehicle brakes on a road that is .mu.-split, meaning it has different coefficients of friction under the left and right sides of the vehicle, a yawing moment occurs that pulls the vehicle in the direction of the side with the higher coefficient of friction. The driver must counteract this effect by steering in the opposite direction. This can be a problem for an inexperienced driver, particularly if the yawing moment occurs suddenly or is especially strong. The yawing moment is generally larger for vehicles that are lightly loaded or have a short wheel base or a wide gauge. It is also harder to control yawing in a vehicle with a wide positive steering radius. Even in the absence of such load and design factors, yawing can be difficult for the driver to control under extreme road conditions such as dry pavement on one side and ice on the other. Yawing is also more exaggerated during especially hard braking, which can be hardest of all when the vehicle is equipped with a regulating anti-lock brake system (ABS).
Special electronic measures that can be taken within an anti-lock brake system (ABS) to counteract yawing are already known from DE-OS 28 55 326 (U.S. Pat. No. 4,349,876). These measures attenuate the yawing moment and make it easier for the driver to adjust to the onset of yawing when braking takes place on a road with different surfaces on the right and left sides of the vehicle. The wheel running on the side of the road with the higher coefficient of friction is called the "high" wheel and the wheel on the other side of the axle is called the "low" wheel. The onset of the yawing moment is controlled by building up the braking pressure in the high wheel with a predetermined time delay. The braking pressure in the high wheel is also prevented from rising above a constant value, thereby relieving the driver of the need to counter an excessive yawing moment.
Lately, vehicles equipped with so-called electrically controlled braking systems (EBS) have also become known. In such braking systems, e.g., the braking system described in DE-OS 44 06 235, the actual value of the braking pressure in a brake cylinder is adjusted to a desired braking value generated by a braking force transmitter. Normally, these EBS braking systems are also equipped with an anti-lock brake system (ABS). Because pressure sensors are used in an electrically controlled braking system (EBS), it is possible to limit the braking pressure difference (.DELTA.P) between the high wheel and the low wheel and thereby reduce the yawing moment. See DE-OS 24 60 309 (U.S. Pat. No. 3,988,043) for another example where braking at the high wheel is co-controlled by the braking at the low wheel, which is controlled directly by an ABS. The pressure sensors of an EBS also make it possible to prevent the pressure at the high wheel from rising above a predetermined value.
Finally DE-PS 28 51 107 (U.S. Pat. No. 4,313,166) discloses a method by which a vehicle with an ABS produces slowly rising, mostly smooth yawing moments on .mu.-split road surfaces. In this method, the braking pressure on the high wheel is maintained at a constant value during the venting of the low wheel until the slip signal (.lambda.) of the low wheel drops. Following this, the high wheel is also vented, and this for a period of time which amounts to a certain fraction of its pressure holding time.
The disadvantage of the known arrangements is that the resulting attenuation of the yawing moment is not adapted in real time to the nature of the vehicle and both sides of the road. First, the braking pressure at the high wheel depends on the braking pressure evolution of the low wheel, and its starting phase is therefore limited. As a result, the maximum admissible braking force of the high wheel that produces a yawing that the driver can control is not always utilized sufficiently. It is also possible that, in some cases, excessive under-braking of the high wheel causes the braking distance to be unnecessarily increased. It is also possible that the predetermined constant pressure difference (.DELTA.P) is too high at times. For example, a lightly loaded vehicle, or a tractor with no trailer, may be difficult or impossible for the driver to control during hard braking on a severely .mu.-split roadway.
It is therefore the object of the present invention to propose a process for attenuating the yawing moment that, on the one hand, relieves the driver from excessive counter-steering and, on the other hand, shortens the braking distance of the vehicle from that of known systems.